The present invention pertains to the refining of metals and alloys with gaseous mixtures, and in particular to a method of refining primary and secondary nonferrous metals and alloys using gases having reduced global warming potentials relative to the prior art.
Prior to casting, a molten metal (pure metal or an alloy) usually contains many impurities, such as dissolved gases, refractory particles, tramp elements and suspended non-metallic particles (e.g., metal oxides). If not removed, these impurities cause high scrap loss in casting, or otherwise cause poor metal quality in products fabricated therefrom.
A variety of refining methods and processes have been developed to remove such impurities from molten metal. Generally, these methods and processes comprise contacting the molten metal with a refining gas. Commonly used refining gases include inert gases (e.g., nitrogen, argon, and mixtures thereof), reactive gases such as sulfur hexafluoride (SF6), halocarbon gases, and chlorine (Cl2), and mixtures thereof. The methods and processes discussed below are representative of the prior art.
U.S. Pat. No. 3,839,019 (Bruno, et al.) discloses a process for removing dissolved hydrogen and aluminum oxide particles from molten aluminum, while purifying the metal of alkali, and alkaline earth elements, such as sodium and calcium, by using a gas mixture of chlorine and argon. The process also is useful in reducing the magnesium content in an aluminum alloy when so desired.
U.S. Pat. No. 3,854,934 (Dore, et al.) discloses a similar process for treating aluminum melts that overcomes the difficulty of separating the liquid chloride salts that form when using chlorine. In this process, a halocarbon gas, such as dichlorodifluoro-methane (CCl2F2), is introduced as a reactive gas beneath a supernatant salt cover to remove the impurities, the resultant complexes of chloride and fluoride salts being more readily separated from the melt. Unfortunately, the carbon present in the reactive gas molecule combines with the aluminum to form aluminum carbide inclusions in the melt. In practice, the halocarbon may also only partially decompose to form tetrafluoro-methane (CF4), which unbeneficially consumes fluorine value.
U.S. Pat. No. 4,392,888 (Eckert, et al.) discloses a process that overcomes this carbide formation problem by exposing the halocarbon to a controlled amount of an oxidizer, such as oxygen. The carbon is preferentially oxidized to carbon monoxide rather than carbon dioxide, as the latter will react with the aluminum to form aluminum oxides. This process can be further improved by also introducing a fluorine acceptor, such as silicon tetrachloride to minimize the formation of CF4.
While the above processes allow the metal to be successfully refined, the use of halocarbon gases is less acceptable from an environmental perspective. The chlorofluorocarbons (CFC""s), like dichlorodifluoromethane (CCl2F2) or CFC-12, and the hydrochlorofluorocarbons (HCFC""s), like monochlorodifluoromethane (CHF2Cl) or HCFC-22, are recognized stratospheric ozone depleting substances and their manufacture and use has been regulated or banned by the EPA under the Clean Air Act in compliance with the Montreal Protocol on ozone depletion. The perfluorocarbons (PFC""s), like tetrafluoromethane (CF4), and the hydrofluorocarbons (HFC""s), like tetrafluoroethane (CH2FCF3) or HFC-134a, generally are recognized as global warming compounds. The EPA similarly restricts their use with a scheduled complete phase out in production.
U.S. Pat. No. 5,145,514 (Gariepy, et al.) discloses a process for removing dissolved alkali metals, alkaline earth metals and hydrogen from molten aluminum-magnesium alloys. The critical aspect of this invention is that fluxing with chlorine is carried out by admixing a fluorine gas of sulfur hexafluoride or silicon tetrafluoride with the chlorine so as to prevent the formation of a brittle and non-protective oxide crust on the surface of the molten alloy.
U.S. Pat. No. 4,350,524 (Knoepke, et al.) discloses a method of refining crude silver to remove base metal impurities (e.g., bismuth, lead, antimony, tin, etc.) by treatment with sulfur hexafluoride gas. The crude silver is melted and maintained at about 1,000_C. while it is treated concurrently with sulfur hexafluoride and an oxidizing gas, such as air. (The SF6 and oxidizing gas are bubbled through the molten silver.) Base metal fluorides are formed and are removed from the silver, after which the molten bath is treated with a reducing agent, such as hydrogen gas, propane, or calcium metal.
U.S. Pat. No. 3,434,825 (Dore, et al.) discloses a process for removing both metallic and gaseous impurities from copper base alloys by bubbling a gaseous monomeric, halogen containing, lower aliphatic hydrocarbon containing at least one fluorine atom, such as dichlorodifluoromethane (CCl2F2), through the copper. The reactive gas is used either pure or as a mixture diluted in an inert.
U.S. Pat. No. 4,010,030 (French) discloses a process for removing metal impurities from molten copper. Bismuth, antimony and arsenic impurities are removed from molten copper by contacting the molten copper with sulfur hexafluoride gas, which reacts directly with the impurities to form gaseous fluorides. An oxygen-containing gas (e.g., air, a mixture of air and oxygen, and/or a mixture of oxygen and an inert gas, such as nitrogen) may be blown through the molten copper simultaneously with or after the SF6 treatment to remove any sulfur contamination from the SF6 treatment.
U.S. Pat. Nos. 3,743,263 (Szekely) and 4,047,938 (Szekely) disclose a process for refining molten metals from the group consisting of magnesium, copper, zinc, tin and lead. The refining gas is argon, nitrogen, or a mixture of argon and nitrogen, all of which are inert towards the molten metal being refined. Hydrogen and carbon monoxide may be used in instances where these gases will not react with the molten metal, but will react with gaseous impurities such as oxygen. Other reactive gases with similar characteristics also can be used, such as sulfur hexafluoride (SF6), chlorine (Cl2), and halogenated hydrocarbons. Selection of a particular refining gas is generally made in accordance with the characteristics of the particular metal being refined.
U.S. Pat. No. 4,992,241 (Provencher, et al.) discloses a process for reprocessing aluminum matrix composites. The process includes treating molten aluminum with a combination of a nonreactive gas and a reactive gas. The reactive gas can be Cl2, SF6 or a mixture thereof. The reactive gas comprises between 2% and 20% by volume of the treatment gas. The patent discloses treatment gas flow rates in the range of 2-6 l/min. for a 300 kg melt.
U.S. Pat. No. 4,959,101 (MacNeal, et al.) discloses a process for degassing aluminum melts without substantially altering the magnesium content of the melt. The process comprises contacting the melt with a mixture of an inert gas (e.g., nitrogen or argon) and a halogenated sulfur compound, such as sulfur hexafluoride (SF6). The sulfur compound is present in the inert gas in an amount between 2% and 20%. The amount of the treatment gas used is disclosed to be about 35 cubic feet per ton of aluminum at a pressure of about 30-45 psig.
Many of the prior art methods and processes use a halocarbon or SF6 as a reactive gas. The use of the reactive gas in a pure form was generally discarded because at the metal melt temperatures the decomposition products cause severe attack on ferrous equipment and non-metallic inclusions. In addition, the use of pure SF6 with molten metals such as magnesium has been reported to have caused explosions. Although sulfur hexafluoride (SF6) and many of the halocarbons are considered physiologically inert, these act as simple asphyxiants which displace oxygen from the breathing atmosphere.
The use of these gases also has some drawbacks. The primary drawback is the release to the atmosphere of material having a high global warming potential (GWP) or an ozone depleting potential.
SF6 has a 100-year global warming potential (GWP100) of 23,900 relative to CO2. International concern over global warming has focused attention on the long atmospheric life of SF6 (about 3,200 years, compared to 50-200 years for CO2) together with its high potency as a greenhouse gas (23,900 times the GWP100 of CO2 on a mole basis) and has resulted in a call for voluntary reductions in emissions. Because of this concern, the use of SF6 is being restricted and it is expected to be banned in the near future. In addition, SF6 is a relatively expensive gas as compared to chlorine and some halocarbon gases.
It is desired to have a process for refining nonferrous metals and alloys which overcomes the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It is further desired to have an improved method of processing nonferrous metals and alloys using refining gases having no or negligible ozone depleting potential and lower global warming potentials than the gases used in prior art methods.
It also is desired to have an improved method of processing nonferrous metals and alloys using refining gases which overcomes the difficulties and disadvantages of the prior art to provide better and more advantageous results.
A first embodiment of the present invention is an improvement in a method for processing a nonferrous metal and alloys of the metal using a refining gas having a global warming potential. The improvement comprises reducing the global warming potential of the refining gas by refining the nonferrous metal and alloys with a gaseous mixture including at least one compound selected from the group consisting of ClF3, ClF, COF2, F3COF, CF2(OF)2, SO2F2, SOF2, SOF4, NF3, SO2ClF, NOF, F2 and SF4.
There are several variations of the first embodiment of the improvement in the method. In one variation, the at least one compound is provided at a first concentration of less than about 10% on a mole basis of said gaseous mixture. In addition, there are several variants of that variation. In one variant, the first concentration is less than about 6%. In another variant, the first concentration is less than about 1%.
In yet another variant, the gaseous mixture further comprises at least one member selected from the group consisting of He, Ne, Xe, Kr, Cl2, N2, Ar, CO, CO2, SO2, O2, and air. In a variant of that variant, the at least one member is provided at a second concentration of about 30% to about 60% on a mole basis. In a variant of that variant, the at least one compound is SO2F2 provided at the first concentration of less than about 3% on a mole basis.
In another variation of the first embodiment of the improvement in the method, the gaseous mixture used in the method further comprises an odorant. In yet another variation, at least a portion of the gaseous mixture is recovered for reuse.
The present invention also includes an improvement in a method for processing a melt comprising at least one molten nonferrous metal using a refining gas having a global warming potential. The improvement comprises reducing the global warming potential of the refining gas by refining the melt with a gaseous mixture including at least one compound selected from the group consisting of ClF3, ClF, COF2, F3COF, CF2(OF)2, SO2F2, SOF2, SOF4, NF3, SO2ClF, NOF, F2 and SF4.
The present invention also includes a method for removing impurities from an ingot of a nonferrous metal and alloys of the metal. The method comprises heating the ingot in a presence of an atmosphere containing an effective amount of at least one compound selected from a group consisting of ClF3, ClF, COF2, F3COF, CF2(OF)2, SO2F2, SOF2, SOF4, NF3, SO2ClF, NOF, F2 and SF4.
The present invention also includes a process for treating a nonferrous metal and alloys of the metal.
A first embodiment of the process includes refining the nonferrous metal and alloys with an atmosphere containing an effective amount of at least one compound selected from the group consisting of ClF3, ClF, COF2, F3COF, CF2(OF)2, SO2F2, SOF2, SOF4, NF3, SO2ClF, NOF, F2 and SF4.
There are several variations of the first embodiment of the process. In one variation, the at least one compound is provided at a first concentration of less than about 10% on a mole basis of the atmosphere. In addition, there are several variants of that variation. In one variant, the first concentration is less than about 6%. In another variant, the first concentration is less than about 1%.
In yet another variant, the atmosphere further comprises at least one member selected from the group consisting of He, Ne, Xe, Kr, Cl2, N2, Ar, CO, CO2, SO2, O2 and air. In a variant of that variant, the at least one member is provided at a second concentration of about 30% to about 60% on a mole basis. In a variant of that variant, the at least one compound is SO2F2 provided at the first concentration of less than about 3% on a mole basis.
In another variation of the first embodiment of the process, the gaseous mixture used in the process further comprises an odorant. In yet another variation, at least a portion of the atmosphere is recovered for reuse.
The present invention also includes a process for refining a melt comprising at least one molten nonferrous metal. The process comprises refining the melt with an atmosphere containing an effective amount of at least one compound selected from the group consisting of ClF3, ClF, COF2, F3COF, CF2(OF)2, SO2F2, SOF2, SOF4, NF3, SO2ClF, NOF, F2 and SF4.